[php-nextgen] Use PHP 8.1 backed enums

[JulianVennen]

In the PHP generator, enums are implemented by creating a class that stores the enum values in static properties.
This results in weird type hints where these enums are used since the PHPDoc type specifies the class, but the actual values are the values of these properties.

PHP 8.1 adds backed enums which allow us to implement this correctly with less confusing type hints and better developer ergonomics.

Since backed enums need to declare the backing type, I introduced the vendor extension x-php-enum-type.
This follows the same principle as how a similar problem was solved in the python generator.

PR checklist

• Read the contribution guidelines.
• Pull Request title clearly describes the work in the pull request and Pull Request description provides details about how to validate the work. Missing information here may result in delayed response from the community.
• Run the following to build the project and update samples:

  ./mvnw clean package 
  ./bin/generate-samples.sh
  ./bin/utils/export_docs_generators.sh
  

  Commit all changed files.
  This is important, as CI jobs will verify all generator outputs of your HEAD commit as it would merge with master.
  These must match the expectations made by your contribution.
  You may regenerate an individual generator by passing the relevant config(s) as an argument to the script, for example ./bin/generate-samples.sh bin/configs/java*.
  For Windows users, please run the script in Git BASH.
• In case you are adding a new generator, run the following additional script :

  ./bin/utils/ensure-up-to-date
  

  Commit all changed files.
• File the PR against the correct branch: master (7.0.1 - patch release), 7.1.x (minor release - breaking changes with fallbacks), 8.0.x (breaking changes without fallbacks)
• If your PR is targeting a particular programming language, @mention the technical committee members, so they are more likely to review the pull request.

@jebentier, @dkarlovi, @mandrean, @jfastnacht, @ybelenko, @renepardon

[wing328]

LGTM but I've not tested it locally.

I'll later PM you on how to add some tests.

Thanks again for the PR.